Not applicable.
The invention relates to a lens with at least one aspheric lens surface, to an objective with at least one aspheric lens surface, and to a projection exposure device for microlithography and a method for the production of microstructured components with an objective having at least one aspheric lens surface.
Lenses with aspheric lens surfaces are increasingly used, particularly in projection objection objectives for microlithography, for improving imaging quality. For example, such projection objectives are known from German Patent Documents DE 198 18 444 A1, DE 199 42 281, U.S. Pat. Nos. 5,990,926, 4,948,328, and European Patent Document EP 332 201 B1.
Aspheric lenses are increasingly used in projection objection objectives for microlithography, for improving imaging quality. However, in order to attain the desired quality improvement by the use of lenses with aspheric lens surfaces, it is necessary that the actual shape of the aspheric lens surfaces does not deviate more than a predetermined amount form the reference data of the lens surface. The permissible deviations between the reference surface and the actual surface are very small in microlithography, because of the finer and finer structures to be imaged. For testing whether a present aspheric lens surface corresponds to the required lens surface within the range of measurement accuracy, a special test optics is required. The quality of the aspheric lens surface is tested with this test optics.
The complexity of such test optics depends definitively on the surface shape of the aspheric lens surface. In particular, the use is desirable of aspheric lenses whose aspheric lens surface can be tested by test optics which can be provided at a justifiable cost and which preferably consists of a small number of spherical lenses.
It can also be necessary in the production of aspheric lens surfaces for the aspheric lens surface to have to be tested and reworked repeatedly during the production process.
Due to polishing also, an undesired and non-uniform change of the surface shape can arise in dependence on the surface because of polishing removal, resulting in an impermissible change in the aspheric lens surface.
Furthermore, it can also happen with aspheric lenses of high asphericity, that is, with a large deviation from a spherical surface, and with a strong variation of the local curvature, that these surfaces can be polished only with very small polishing tools, with a very large polishing cost, or it is nearly impossible to polish the aspheric surface. Just in the process of designing objectives, it is not comfortable if the designer can only find out, by multiple consultations with the polishing specialist and with the specialist responsible for preparing the test optics, whether a design he has developed can be manufactured, or whether he has to change the design, so that a design exists which is also acceptable from manufacturing standpoints. Particularly when manufacture and development are spatially separated from one another, discussion and agreement between design and manufacturing entails a considerable cost in time.
The invention has as its object to provide a method by which new designs with aspheric lens surfaces can be generated without consultation with manufacturing.
The object of the invention is attained by the following features: By the measure of describing the aspheric lens surfaces by Zernike polynomials, it is possible to undertake a classification of aspheric lens surfaces such that the respective aspheric lens surface can be polished and tested at a justifiable cost when at least two of the three conditions (a)-(c) according to the following conditions are present:                               P          ⁡                      (            h            )                          =                ⁢                                            h              2                                      R              (                              1                +                                                                            1                      -                                                                        h                          2                                                                          R                          2                                                                                      )                                                                                +          K0          +                      K4            *            Z4                    +                      K9            *            Z9                    +                      K10            *            Z16                    +                                                ⁢                              K25            *            Z25                    +                      K36            *            Z36                    +                      K49            *            Z49                    +                      K64            *            Z64                              
with
Z4=(2xc3x97h2xe2x88x921)
Z9=(6h4xe2x88x926h2+1)
Z16=(20h6xe2x88x9230h4+23h2xe2x88x921)
Z25=(70h8xe2x88x92140h6+90h4xe2x88x9220 h2+1)
Z36=(252h10xe2x88x92630h8+560h6xe2x88x92210h 4+30 h2xe2x88x921)
Z49=(924h12xe2x88x9227.72h 10+h3150h8xe2x88x921680h6+h420h4xe2x88x9242h2+1)
Z64=(3432h14xe2x88x9212012h12+16632h110xe2x88x92h11550h8+4200h6xe2x88x92756h4+56h2xe2x88x921)
where P is the sagitta as a function of the normed radial distance h from the optical axis 7:       h    =                            distance          ⁢                      xe2x80x83                    ⁢          from          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          optical          ⁢                      xe2x80x83                    ⁢          axis                                      1            2                    ⁢                      (                          lens              ⁢                              xe2x80x83                            ⁢              diameter              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              the              ⁢                              xe2x80x83                            ⁢              aspheric                        )                              =              normed        ⁢                  xe2x80x83                ⁢        radius                  0     less than     h    ≤    1  
xe2x80x83and wherein at least two of the following conditions is fulfilled:                               "LeftBracketingBar"                      K16            K9                    "RightBracketingBar"                 less than         0.7                            (        a        )                                          "LeftBracketingBar"                      K25            K9                    "RightBracketingBar"                 less than         0.1                            (        b        )                                          "LeftBracketingBar"                      K36            K9                    "RightBracketingBar"                 less than         0.02                            (        c        )            
xe2x80x83the radius of the aspheric lens surface being fixed so that K4=0.
The object of the invention is also achieved when all of the above conditions (a through c) are fulfilled.
Thus it is possible for the designer, without consultation with manufacturing, to be able to make a statement about whether his design can be tested and produced. The designer can limit himself to producing designs which can be tested and manufactured.
In particular, the presence of condition (c) has an advantageous effect on the manufacturability of aspheric lens surfaces. By the measure that the proportions resulting from the Zernike polynomial, relative to the normal radius, do not exceed the following contributions, a class of aspheric lens surface is created which are outstanding for easy manufacturability and testability. Those contributions are:
By analogy to a vibrating air column or vibrating string, the coefficients Z16, Z25, Z49, Z64, etc. could be described as the overtones of the aspheric object. The poorer in overtones, i.e., the faster the decay of the amplitudes of the components from the Zernike polynomials Z16 and greater, the easier it is to manufacture an aspheric. Furthermore, a compensation optics having lenses, or a computer-generated hologram, for testing the aspheric thereby becomes substantially insensitive as regards tolerances. In addition, rapid decay of the amplitudes makes it possible to find an isoplanatic compensation optics. The natural decay of the amplitudes of the Zernike contributions is decisive for the quality of matching of the test optics to the aspheric lens surface (residual RMS value of the wavefront). This is clear from the example put forward, with a particularly harmonic decay of the higher Zenike amplitudes. It would also be undesirable to unnaturally decrease an individual higher Zernike term in its amplitude. A compensation optics of spherical lenses with a technically reasonable sin-i loading generates quite by itself a gently decaying amplitude pattern of higher Zernike terms.
It has furthermore been found to be advantageous to provide the aspheric lens surface on a convex lens surface. This has an advantageous effect on the polishing process.
It has been found to be advantageous to provide in an objective only aspheric lens surfaces which according to the characterization by Zernike polynomials are easily produced with the required accuracy.
It has been found to be advantageous, in order to further improve the effect of these aspheric lens surfaces, to arrange a spherical lens surface respectively neighboring the aspheric lens surface and having a radius which deviates at most by 30% from the radius of the aspheric lens surface. By this measure, a nearly equidistant air gap is formed between the aspheric lens surface and the adjacently arranged spherical lens surface. The designer is thereby freer in the curvature of the aspheric, which represents an additional important degree of freedom of the aspheric, without thereby making it difficult to manufacture the aspheric.